Natural gas typically is transported from the location where it is produced to the location where it is consumed by a pipeline. However, large quantities of natural gas may be produced in a country in which production by far exceeds demand. Without an effective way to transport the natural gas to a location where there is a commercial demand, the gas may be burned as it is produced, which is wasteful.
Liquefaction of the natural gas facilitates storage and transportation of the natural gas. Liquefied natural gas (“LNG”) takes up only about 1/600 of the volume that the same amount of natural gas does in its gaseous state. LNG is produced by cooling natural gas below its boiling point (−259° F. at ambient pressures). LNG may be stored in cryogenic containers either at or slightly above atmospheric pressure. By raising the temperature of the LNG, it may be converted back to its gaseous form.
The growing demand for natural gas has stimulated the transportation of LNG by special tanker ships. Natural gas produced in remote locations, such as Algeria, Borneo, or Indonesia, may be liquefied and shipped overseas in this manner to Europe, Japan, or the United States. Typically, the natural gas is gathered through one or more pipelines to a land-based liquefaction facility. The LNG is then loaded onto a tanker equipped with cryogenic compartments (such a tanker may be referred to as an LNG carrier or “LNGC”) by pumping it through a relatively short pipeline. After the LNGC reaches the destination port, the LNG is offloaded by cryogenic pump to a land-based regasification facility, where it may be stored in a liquid state or regasified. To regasify the LNG, the temperature is raised until it exceeds the LNG boiling point, causing the LNG to return to a gaseous state. The resulting natural gas then may be distributed through a pipeline system to various locations where it is consumed.
For safety, ecological, and/or aesthetic considerations, it has been proposed that regasification of the LNG take place offshore. A regasification facility may be constructed on a fixed platform located offshore, or on a floating barge or other vessel that is moored offshore. The LNGC may be either docked or moored next to the offshore regasification platform or vessel, so that LNG may then be offloaded by conventional means for either storage or regasification. After regasification, the natural gas may be transferred to an onshore pipeline distribution system.
It also has been proposed that regasification take place onboard the LNGC. This has certain advantages, in that the regasification facility travels with the LNGC. This can make it easier to accommodate natural gas demands that are more seasonal or otherwise vary from location to location. Because the regasification facility travels with the LNGC, it is not necessary to provide a separate LNG storage and regasification facility, either onshore or offshore, at each location at which LNG may be delivered. Instead, the LNGC fitted with regasification facilities may be moored offshore and connected to a pipeline distribution system through a connection located on an offshore buoy or platform.
When the regasification facility is located onboard the LNGC, the source of the heat used to regasify the LNG may be transferred by use of an intermediate fluid that has been heated by a boiler located on the LNGC. The heated fluid may then be passed through a heat exchanger that is in contact with the LNG.
It also has been proposed that the heat source be seawater in the vicinity of the LNGC. As the temperature of the seawater is higher than the boiling point of the LNG and the minimum pipeline distribution temperature, it may be pumped through a heat exchanger to warm and regasify the LNG. However, as the LNG is warmed, regasified, and superheated, the seawater is chilled as a result of the heat transfer between the two fluids. Care must be taken to avoid cooling the seawater below its freezing point. This requires that the flow rates of the LNG being warmed and the seawater being used to warm the LNG be carefully controlled. Proper balancing of the flow rates is affected by the ambient temperature of the seawater as well as the desired rate of gasification of the LNG. Ambient temperature of the seawater can be affected by the location where the LNGC is to be moored, the time of year when delivery occurs, the depth of the water, and even the manner in which the chilled seawater from warming the LNG is discharged. Moreover, the manner in which the chilled seawater is discharged may be affected by environmental considerations, e.g., trying to avoid an undesirable environmental impact such as ambient water temperature depression in the vicinity of the chilled seawater discharge. Environmental concerns can affect the rate at which the LNG may be heated, and, therefore, the volume of LNG that can be gasified in a given period of time with regasification equipment on board the LNGC.